(1) Field of the Invention
The present invention relates to an LED current balance apparatus, and more particularly relates to an LED current balance apparatus with current-balancing capacitive unit.
(2) Description of the Prior Art
Due to process errors of light emitting diodes (LEDs), forward voltages of the LEDs are different. Especially, LED strings with a plural LEDs connected in series have serious voltage difference of total forward voltages. Therefore, when a plural LED strings are driven by the same voltage source, the voltage difference of total forward voltages have to be compensated for balancing LED currents.
FIG. 1 is a detailed circuit diagram of a current balancing apparatus provided by Samsung Electronics Co., Ltd. A switching mode power supply comprises two switches S1 and S2, a capacitor Ca, a transformer, diodes D1-D12, balancing capacitors Cb1, Cb2 and Cb3, and capacitors C1-C6. The switching mode power supply converts an input current into an alternating current, which is rectified by the diodes D1-D12 and filtered by the capacitors C1-C6 to light LEDs Ld1-Ld6. The transformer has a primary coil L1 and three secondary coils L21, L22, L23, and the balancing capacitors Cb1, Cb2, Cb3 are coupled to corresponding secondary coils L21, L22, L23 for compensating impedance differences of two LEDS of respective secondary coils. An impedance value of a capacitor is determined by operating frequency and capacitance. The impedance values of the balancing capacitors Cb1, Cb2 and Cb3 in the current balancing apparatus is set to be much higher than the impedance values of the LEDs Ld1-Ld6, i.e., an impedance value seen by the balancing capacitor looking toward the diode is negligible compared to an impedance value of the balancing capacitor. Therefore, the same DC current is applied to the LEDs Ld1-Ld6, and so the current balancing of the LEDs Ld1-Ld6 is accomplished.
However, the LED driving system still has some problems. Firstly, the currents of the LEDs Ld1-Ld6 cannot be stabilized at an excepted current value. It results from that the balancing capacitors Cb1, Cb2 and Cb3 have tolerances in capacitance and vary in capacitance with temperature. Moreover, the actual operating frequency of the current balancing apparatus may not be a preset frequency and so affects the amount of the current of the LEDs Ld1-Ld6. Secondly, the currents of the secondary coils of the transformer are sine waves before being rectified and so the amplitudes of the currents are periodically varied in response to the switching of the switches S1 and S2. Even after the currents are rectified by the capacitors C1-C6, the currents provided by the secondary coils L21, L22, L23 to the flowing through the LEDs Ld1-Ld6 still have large current ripples due to the LEDs Ld1-Ld6 with an end grounded having low impedance values. Thirdly, the LEDs Ld1-Ld6 can be dimmed by cutting off the switches S1 and S2 to stop a power conversion of the switching mode power supply. However, when cutting off the switches S1 and S2, the LEDs Ld1-Ld6 still lights for a certain time period due to the energy stored in the transformer, the balancing capacitors Cb1, Cb2 and Cb3, and the capacitors C1-C6, and then stops lighting. Furthermore, during the certain time period, the currents of the LEDs Ld1-Ld6 are decreased with time, which cause color shift. When the switches S1 and S2 are conducted again, the LEDs Ld1-Ld6 begin to light after a certain time period of recovering the stored energy of the transformer, the balancing capacitors Cb1, Cb2 and Cb3, and the capacitors C1-C6. Similarly, during the certain time period, the currents of the LEDs Ld1-Ld6 are increased with time, which still cause color shift. Thus, this type of the LED driving system has problems of inaccuracy dimming control and color shift.